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Sex Allocation Ratios in the Facultatively Polygynous Ant, Leptothorax Acervorum

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Sex Allocation Ratios in the Facultatively Polygynous Ant, Leptothorax Acervorum View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by The University of North Carolina at Greensboro Sex allocation ratios in the facultatively polygynous ant, Leptothorax acervorum By: J. Heinze, A. Hartmann, and O. Rüppell Heinze J., Hartmann A. & Rüppell O. (2001) Sex allocation ratios in the facultatively polygynous ant, Leptothorax acervorum. Behavioral Ecology and Sociobiology 50: 270- 274. Made available courtesy of Springer Verlag: http://www.springerlink.com/content/rre9gr53vbqr3xye/?p=effc6907a317468d9186cd5f5f9e369 8&pi=8 The original publication is available at www.springerlink.com ***Note: Figures may be missing from this format of the document Abstract: We investigated sex allocation in a central European population of the facultatively polygynous ant Leptothorax acervorum. The population-wide sex ratio was found to be quite balanced, with a proportional investment in female sexuals of 0.49. Sex allocation varied considerably between colonies, resulting in split sex ratios. The productivity of colonies was negatively correlated with queen number and positively with colony size. In contrast, the sex ratio (proportional investment in female sexuals) was neither correlated with queen number, colony size, nor total sexual production, but with worker relatedness. The uncoupling of the genetic colony structure and queen number presumably results from frequent queen turnover and colony splitting. Keywords. Sex ratio - Relatedness - Relative relatedness asymmetry - Polygyny – Formicidae Article: Introduction Populations of many animals consist of equal numbers of males and females. The occurrence of balanced sex ratios in some species with environmental sex determination (e.g., Bull and Charnov 1988), and skewed sex ratios in birds with sex chromosomes (e.g., Komdeur et al. 1997) indicates that this 1:1 sex ratio is frequently not a simple consequence of the mechanisms of sex determination. Instead, Fisher (1930) argued that a population-wide 1:1 sex investment ratio is an evolutionarily stable strategy, because only then do the individuals that control the investment obtain equal fitness payoffs from both sexes. Sex allocation in social Hymenoptera (bees, wasps, and ants) is of special interest, because the sex investment of a colony can be controlled by two parties, the queen and her helping daughters (workers), whose fitness interests may differ: the mother queen is equally related to her sons and daughters, but workers in colonies with one singly mated queen (monogyny, monandry) are three times more closely related to female sexuals than to males (relatedness asymmetry). Whereas the queen therefore favors equal investment in male and female sexuals, workers benefit most from investing three times more in female than male sexuals. The finding that, on average, population- wide sex investment ratios in monogynous, monandrous ants are female-biased, strongly suggests worker control (Trivers and Hare 1976; Bourke and Franks 1995; Crozier and Pamilo 1996; Chapuisat and Keller 1999). However, the pattern of sex investment in social Hymenoptera is often more complex. For example, in many species, colonies are not obligatorily monogynous but some nests may have several related queens (facultative polygyny). The workers' relatedness asymmetry at the colony level is expected to decline with increasing queen number per colony and, as a consequence, a negative correlation of colony queen number and female sex investment is predicted under the relative relatedness asymmetry hypothesis (Boomsma and Grafen 1990, 1991). Several studies have clearly demonstrated this effect (Boomsma 1993; Bourke and Franks 1995; Crozier and Pamilo 1996). Leptothorax acervorum is a facultatively polygynous ant, whose colonies consist only of a few dozen workers which nest in rotting branches and under bark. Queens typically mate singly (Heinze et al. 1995), and in colonies containing a queen, workers rarely lay eggs and, therefore, presumably do not contribute considerably to the colonies' males (Bourke 1991; Heinze et al. 1997a). Previous investigations revealed marked differences in sex allocation patterns between populations of this ant: in an English population (Chan and Bourke 1994; Chan et al. 1999), an effect of queen number on colony sex ratio was demonstrated, and allozyme data suggested that virgin queens in the Reichswald population in southern Germany are predominantly reared in nests with high relatedness (Heinze et al. 1995). Here, employing high-resolution genetic markers (microsatellites), we show that sex investment ratios in the Reichswald population vary with average worker relatedness but not with queen number, presumably due to rapid changes in the social structure of colonies. Methods Colonies of L. acervorum were collected from their nests in rotting pine twigs in an open pine forest, in Nürnberger Reichswald near Erlangen in early summer 1998. Colonies were kept in the laboratory as previously described (Heinze and Ortius 1991) until no more sexuals eclosed from the brood. New workers eclosed over a prolonged period of time and their production was presumably influenced by rearing conditions in the laboratory. Hence, the total productivity of colonies was not examined. The dry weights of female and male sexuals of L. acervorum do not differ significantly (Chan and Bourke 1994; Heinze et al. 1995), so the sex ratio (defined as the proportional investment in virgin queens) was therefore estimated as the ratio of female sexuals to the total number of sexuals. We investigated the effects of parameters which have previously been discussed as influential on sex ratio (queen number, worker number, and total sexual production) and total sexual production (queen number and worker number). The distribution of data was normalized by log10(x+0.5) transformation (Kolmogorov-Smirnov test; queen number, D=0.167, P>0.20; worker number, D=0.054, P>0.20; total sexual production, D=0.119, P>0.20) and angular transformation (sex ratios, D=0.142, P>0.20). Relatedness was normally distributed (D=0.141, P>0.20). Transformed data were analyzed by parametric multiple regression. Because of the presumed interrelations between the various "independent variables," in addition we analyzed the data by standard methods (James and McCulloch 1990), employing bootstrapping for significance tests (Manly 1997; O. Rüppell, J. Heinze, B. Hölldobler, unpublished data). DNA for genetic analyses was extracted from adult ants using Chelex 100 following a protocol by Thorén et al. (1995). Microsatellites L18 (Foitzik et al. 1997) and LXGT 223 (Hamaguchi et al. 1993) were amplified in 25-µl reaction volumes [5.0 µl DNA, 2.5 µl 10xPCR buffer (MBI Fermentas), 16.7 pmol of each primer, 4.5 µl 4 mM dNTPs, 6.55 µl distilled water, 2.25 µl 25 mM MgCl2, and 0.2 µl (5 units/µl) Taq polymerase (MBI Fermentas)] using a Perkin Elmer GeneAmp 2400 or MWG Primus 96plus thermocycler. The temperature profile started with 5 min at 94°C, followed by 32 cycles of 1 min 94°C, 1 min 54°C, and 1 min 72°C, and ended with 7 min at 72°C. PCR products were separated on 40-cm-long non-denaturing 10% polyacrylamide gels for 3 h at 40 W, stained with Sybr Green (5 µl in 50 ml 1xTBE buffer), and visualized under UV light. Allele size was determined by comparison with a pUC19 DNA/MspI (HpaII) size marker (MBI Fermentas). Within-colony worker relatedness was calculated from microsatellite genotypes with RELATEDNESS 4.2 (K.F. Goodnight and D.C. Queller, based on Queller and Goodnight 1989) and used as an indicator for worker relatedness asymmetry because direct determination of the latter was precluded by low overall sexual production. Results Of 62 collected colonies, 46 were polygynous (median three queens, range two to ten), 15 were monogynous, and 1 colony was queenless. The number of workers per colony ranged from 12 to 157 (median 42.5). Ten colonies did not produce sexuals. In the remaining 52 colonies, a total of 287 virgin queens and 298 males were produced (proportion of females 0.49, confidence interval 0.39-0.59, estimated following Bourke and Franks 1995). The colony-level proportional investment in female sexuals was bimodally distributed (Fig. 1). Twelve colonies (23%) produced only female sexuals (median 2.5, range 1-59) and 13 colonies (25%) produced only male sexuals (median 5, range 1-23). A total of 102 female sexuals (35%) and 105 males (35%) were produced in colonies with single-sex broods. Fig. 1. Between-colony variation in proportional investment in female sexuals in the Reichswald population of the facultatively polygynous ant, Leptothorax acervorum. Proportional investment is bimodally distributed, with some colonies producing only male sexuals and others only female sexual In a multiple-regression analysis, log-transformed total sexual production was found to be positively correlated with log-transformed worker number and negatively with log-transformed queen number (Table 1). Worker number and total sexual production were also correlated in a univariate analysis, whereas the correlation between queen number and total sexual production was not significant (Table 1). Sex ratio (proportional investment in female sexuals) neither varied significantly with queen number, worker number, nor total sexual production (Table 1). Monogynous and polygynous colonies did not differ in their mean proportional investment in females (36 polygynous colonies: median 0.333; 15 monogynous colonies: 0.568, Mann-Whitney U-test, U=247.5, P=0.64). For investigation of within-colony worker relatedness, only colonies that had produced more than ten sexuals were included (5 monogynous, 14 polygynous, 1 queenless). Two colonies in this subset had produced only female sexuals (range 19 to 59) and four only male sexuals (range 15 to 23). Genetic analysis of a total of 262 workers (10-20 workers per colony) revealed considerable variability at both investigated loci (L18: five alleles, Hexp=0.709; LXGT 223: nine alleles, Hexp=0.819). The mean inbreeding coefficient averaged over both loci was 0.020±0.045 (SE), suggesting random mating. The average intracolony relatedness among workers was 0.516±0.051.
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